For an open-plan room that is 21 ft × 35 ft with an 8 ft ceiling, the floor area is:
- 21 × 35 = 735 square feet
- Ceiling height: 8 ft (standard)
A quick sizing estimate for a well-insulated home is about 20–30 BTU per square foot, depending on climate, insulation, windows, and sun exposure.
- 20 BTU/sq ft: ~14,700 BTU
- 25 BTU/sq ft: ~18,400 BTU
- 30 BTU/sq ft: ~22,100 BTU
For a Mitsubishi ductless mini-split, the likely choices are:
- 18,000 BTU (1.5 ton): Suitable if the room is well insulated, has average windows, and is in a moderate climate.
- 24,000 BTU (2 ton): Better if the room has large windows, significant afternoon sun, high ceilings (not in this case), poor insulation, or is in a hot climate.
My recommendation
For a 735 sq ft open-plan living area, a 24,000 BTU Mitsubishi indoor unit is generally the safer choice. Many installers would select a 24k unit because open living/dining/kitchen areas often have:
- Heat from cooking and appliances.
- Multiple occupants.
- Large window areas.
- Air movement into adjacent spaces.
However, if this is a newer, well-insulated home with modest window area and you're in a mild climate, an 18,000 BTU variable-speed Mitsubishi may still be sufficient because inverter systems can ramp their output up and down efficiently.
To size it more accurately, I would need:
- Your location (city/state or climate zone).
- Whether this is primarily cooling, heating, or both.
- Approximate window size and orientation (north/south/east/west).
- Insulation level (new construction vs. older home).
- Whether the kitchen is included in the 21 × 35 ft space.
With that information, I can estimate the load much more closely and recommend the appropriate Mitsubishi model size.
